Lessons 9–10 · 2 lessons · O. Mavlonov. Biology (Zoology), Grade 7. Revised and expanded fourth edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
9
Flatworms: class Turbellaria (ciliated worms)
Textbook: pp. 31–33
GoalExplain the features of flatworms and the structure of ciliated worms using the white planarian.
New words
bilateral symmetry · ikki yonlama simmetriyaciliated epithelium · kiprikli epiteliyhermaphrodite, having both male and female organs · germafroditcocoon holding the eggs · pilla
Explanation
Flatworms are many-celled animals with bilateral symmetry; their digestive, excretory, sensory and hermaphrodite reproductive organs are developed. The phylum splits into three classes: ciliated worms, flukes and tapeworms. The white planarian (2–3 cm) lives on river and lake bottoms under stones and leaves; its body is covered with ciliated epithelium and it glides by its cilia. The mouth is on the underside and the pharynx joins a branched gut, with no hind gut or anus. It is a predator: it covers small aquatic worms and crustaceans with its body and sucks them in with its pharynx. Excretory tubules remove excess water and harmful substances, while nerve ganglia and nerves, sensory tentacles and eyespots detect stimuli. Planarians are hermaphrodites: fertilised eggs are laid in a cocoon; their regeneration is very strong. The oldest flatworms are considered to be the gutless ciliated worms.
Worked examples
The two dark spots are the planarian’s eyespots; they tell light from shadow. A planarian avoids light and hides under stones.
Because gut branches spread through the body, the planarian has no circulatory system: digested food reaches every part along the gut branches.
Class activity
Model a planarian from paper or twigs: head end, eyespots, pharynx, gut branches. Safety: collect from natural water only with adults, never taste the water, and wash hands with soap afterwards.
Practice
1
If we cut a planarian along a line from head end to tail end, what two parts do we get? What symmetry is this?
Matching right and left halves; this is bilateral symmetry.
2
How does a planarian feed and where do the remains leave?
It covers prey and sucks it in with the pharynx; remains leave through the mouth because there is no anus.
3
What is a hermaphrodite?
An animal whose body has both female and male reproductive organs.
4
Why can a planarian break into pieces in bad conditions?
Its regeneration is strong: each piece can regrow into a new animal when conditions improve.